Digestion with alkali metal hydroxide and carbonyl sulfide



United States Patent ()fiice 3,318,761 Patented May 9, 1967 3,318,761 DIGESTION WITH ALKALI METAL HYDROXIDE AND CARBONYL SULFIDE Edward A. Swakon, Hammond, Ind, assignor to Standard Oil Company, Chicago, 11L, a corporation of Indiana No Drawing. Filed Dec. 18, 1963, Ser. No. 331,374 5 Claims. (Cl. 162-82) This invention relates to an improved process of wood pulping and more specifically pertains to a method for obtaining higher yields of wood pulp from wood chips.

Wood pulps are generally obtained from wood chips of soft and hard woods by subjecting the wood chips to the action of aqueous sodium hydroxide at a temperature of from 150 to 180 C. Such a process is known as the soda process. The modification of this process known as the kraft process uses in addition to aqueous sodium hydroxide a solution of sodium sulfide. In either process the mixture of wood chips and caustic or caustic plus sodium sulfide is cooked for several hours at a temperature in the range of 150* to 180 C. Thereafter the slurry of wood pulp in the pulp liquor is subjected to methods for separating solids from liquids, for example, decantation, filtration, separation by centrifugal force in a centrifugal filter or separator. The recovered wood pulp or slurry thereof in fresh water may be subjected to bleaching processes if a light to white wood pulp is desired. Generally the wood pulp from the foregoing pulping processes will vary in color from a tan to brown. Such wood pulp is satisfactory for the manufacture of tan to brown papers used for bags and/ or wrapping paper. The bleached wood pulp, light tan to off-white to white, is used for lighter products including personal tissues, paper towels, news print or in admixture with other fibers to make writing and typing papers.

It has now been discovered that higher yields of wood pulp can be obtained from either the soda process or the kraft process by using in addition to the aqueous caustic or aqueous caustic and sodium sulfide as cooking liquors carbonyl sulfide. When wood chips are cooked at 150 to 180 C. in the presence of aqueous caustic, 1 to sodium hydroxide, or aqueous caustic solutions of the same concentration range also containing sodium sulfide in the presence of carbonyl sulfide, substantially higher yields of wood pulp are obtained than when the wood chips are digested by the soda process or the kraft process without added carbonyl sulfide. For example, by digesting soft wood chips (poplar or sugar pine) by the soda process and the kraft process, pulp yields of 30 to 50% are obtained. The digestion of the same wood chips by modifying the soda process or the kraft process by the addition of carbonyl sulfide thereto increased yields of wood pulp of from 50 to 70% are obtained.

The process of this invention is not limited to the digestion of soft woods to obtain wood pulp but is equally applicable to the digestion of hardwood chips to obtain improved yields of wood pulp.

In the research investigation of the effect of carbonyl sulfide modification of the commonly used soda and kraft wood pulping processes, the following general conditions of the soda and kraft process were followed.

Wood pulping consists of cooking the raw material, usually wood, in suitable chemicals in a digester (reactor) under controlled conditions of temperature, pressure, time and liquor composition. Three chemical processes presently used are the soda, sulfate or kraft, and sulfite. 1n the soda process, wood is cooked with dilute (10%) sodium hydroxide solution at -180 C. for 2-12 hours. In the kraft process, Wood chips are cooked with a solution of sodium hydroxide and sodium sulfide and the yields are slightly higher than in the soda process. Our criteria of evaluating carbonyl sulfide in pulping were to compare pulp appearance and yields with pulp produced under the same conditions in the soda and kraft processes. Tables I and II give the pulp-making conditions, yields and appearance of pulp by the soda, kraft and the new carbonyl sulfide process.

The carbonyl sulfide process consists of cooking Wood chips with carbonyl sulfide and sodium hydroxide solution. The pulping processes were conducted for 4 to 5 hours duration; two hours to reach the maximum temperature of C., and two hours at reaction temperature. A 300 ml. stainless steel autoclave served as the digester.

The pulp yields were appreciably higher in the carbonyl sulfide process than those obtained in the soda or kraft process. The chemical levels used were generally much higher than normally encountered in practice. Pulp yields of 32 to 50% were obtained in the soda and kraft processes and 50-70% in the carbonyl sulfide process. Both hardwood and softwood may be pulped by the carbonyl sulfide process. The color of the pulp, however, was much darker brown. What the effect of this darkercolored pulp is on subsequent papermaking steps is not known. No attempt was made to improve the color of the pulp. It was observed that the higher the reaction temperature, the darker the pulp and that the liquors were black when still highly alkaline but brown when nearly neutral and when excess carbonyl sulfide was used.

Carbonyl sulfide apparently ties up some of the sodium hydroxide. This results in a much milder pulping process and higher pulp yields. In the first step, one mole of carbonyl sulfide reacts with two moles of sodium hydroxide to produce disodium monothiolcarbonate:

At reaction temperature, the disodium monothiolcarbonate could break down into sodium sulfide and carbon dioxide:

A NaOCOSNa NazS +60:

The sodium sulfide thus produced in situ would be a modification of the kraft process. The carbon dioxide liberated also could react further with one or two additional moles of sodium hydroxide. Similar results can be obtained by starting with preformed disodium monothiolcarbonate and by heating wood and sodium hydroxide containing sulfur with carbon monoxide.

Carbonyl sulfide in wood pulping Carbonyl sulfide process of wood pulping.-The standard procedure used to make wood pulp with sodium hydroxide and carbonyl sulfide is as follows. A 300 ml. stainless steel reactor was filled with 20-25 g. of wood chips (poplar or sugar pine). These chips were obtained by cutting a board into Ms- A slits and breaking them into 1 to 1 /2 inch long pieces. 120 g. of aqueous sodium hydroxide solution (1 to 10%) is poured over the chips. The reactor is sealed and carbonyl sulfide is metered into the reactor from a Ruska pump. The reactor with rocking is heated slowly to 170 C.; it generally took 1 /2 hous to 2 hours to reach this temperature. The reactor is kept at this temperature for 2 hours, quenched in cold water, and discharged. The cooked wood is separated from the dark liquid by filtering with a Buchner funnel. The wood chips were transferred to a Waring Blendor and warm water added (150-400 ml.) and blended for -10 minutes. Product filtered again. The washing, blending and filtering was repeated twice more. Wood pulp was dried overnight in an oven at 180 C. and Weighed. Results are given in Table I and Table II.

Soda process of wood prulping.The above procedure was repeated except no carbonyl sulfide was added. Results are given in Table I and Table II.

Kraft process of wood pulping.-The above procedure was repeated except sodium sulfide was dissolved in the sodium hydroxide solution and no carbonyl sulfide was added. Results are given in Table I and Table II.

ventional wood pulping processes demonstrate the advantages obtainable by the method of this invention. These illustrations of the method of this invention and its disclosure will provide for those skilled in the art of wood pulping the guides for applying this new method to the pulping of other woods to obtain the advantages hereinbefore described and illustrated taking into account the art recognized differences known to exist from wood to wood in the conventional wood pulping processes. Accordingly, the method of this invention is not limited to the use of wood chips from one or more illustrated species of trees, but, as those skilled in the art will readily perceive, is widely applicable to the pulping of chips from all wood species.

The foregoing examples illustrate the wood pulping method of this invention with conventionally used sodium hydroxide. However, as those skilled in the art recognize other alkali metal hydroxides such as potassium hydroxide can also be used. The other alkali metal hytion for the use of only sodium hydroxide is not critically limited thereto. Also for the same reason other alkali TABLE I.-CARBONYL SULFIDE PULPING PROCESS CONDITIONS COMPARED TO SODA AND KRAFT PROCESS Chemical to Wood Ratio Added Ingredient Percent Cone. COS Added, Ex. NaOH ml.

Percent Percent Compound Grams Caustic Total 1 Sugar Pine 10 50 50 None No 10 50 120 5 30 8. 3 50 70 5 30 49 8. 3 50 62. 5 8. 3 50 88 8. 3 50 74 2. 5 15 55 NazS-9HzO Sodium monothiol- 3 carbonate. 10 10 0 Sodium carbonate Sodium monothiol- 12 carbonate.

Po lar 29 5 .2 3O 5 31 5 32 5 34 4 35 4 Sulfur and CO 19 36 4 Sulfur 19 37 Sodium Formate- 8. 2 38 1 do 8.2

1 Percent; total is the total amount of chemicals added, including sodium hydroxide, carbonyl sulfide sodium sulfide, etc. and is based on the weight of wood charged.

2 Carbonyl sulfide added after reaction temperature reached C. 3 Carbonyl sulfide added after one-halt hour at 170 C. 4 Carbonyl sulfide added at end of cooking period.

5 Preformed disodium monothiolearbonate was used and because it was introduced as a paste, the weight is not accurately known.

6 Carbonyl sulfide was produced in situ from the reaction of carbon monoxide and sulfur. No'rE.Examples 1, 2 and 35 carried out at F; Examples 21, 25 and 26 carried out at 150 F.; all other examples carried out at F.

The foregoing illustrative exemplifications of the methmetal sulfides such as potassium sulfide can be used in 0d of this invention and the comparison of it with con- 75 the kraft process of wood pulping.

droxides can also be used in the method of this inven- TABLE II.-CARBONYL SULFIDE PULPING PROCESS RESULTS COMPARED TO SODA AND KRAFT PROCESS Pul Yiel Percent Appearance of Pulp Remarks comma cuameemu qmnecncowcswmmm cn gogo wogogouoppmcncngcnmocnmm a: cream or men 001 cu Oooocuqmmcne-mbv oopoocnmcmo m Slate grey-Fine fibers Brown matted fiber Brown tough fibers Grey paper-like pulp--. Reddish brown fibers lrlightlgrgy-tan pullp ery 1g grey pu p Yellow brown Light grey long fiber- Light tan fibers.

Dark brown fibers. Light brown fibers. Brown light density pulp. White pulp Light tan fine fibers Light grey pulp Brown fine pulp Brown fibrous pulp Very dark brown... Tan, coarse fibrous pu p Ofi-white pulp Broylvn fiber pulp o Light brown fiber pu1p Light brown woody pulp- Brown woody materiaL- Woody material. Brown pulp Dark brown pulp. Light tan Brown short fibered pu p. Dark brown pulp gighttan pulp Wood appeared to be overcooked.

Heated for 7 hrs. at 160 0 Compare with Ex. 4. Compare with Ex. 3.

COS had little effect; apparently not enough.

Uncooked wood. Kraft process.

Kra t process. A little undercooked.

Too high a chemical ratio to wood chips.

Standard for soda process. Repeat of Ex. 10.

Repeat of Ex. 16, except COS added at end of cooking period.

Undercooked. Addition of Na;

S did not improve yield or appearance over Ex. 6 and 4. Preformed sodium monothiolcarbonate.

Slightly undercooked. Uudgrcooked. 0.

Standard soda process. Excess COS-dark brown pulp.

Standard soda process. COS prepared in situ. About same yielded 34. Chips slightly undercooked.

Repeated Ex. 37 plus 1% caustic soda.

What is claimed is:

1. The method of preparing wood pulp which comprises digesting wood chips in an aqueous solution of bonyl alkali metal hydroxide which also contains carbonyl sul- 2. The method of claim 1 wherein the aqueous alkali metal hydroxide solution also contains alkali metal sulfide.

3. A method of preparing wood pulp which comprises digesting wood chips in an aqueous solution of sodium hydroxide and which also contains carbonyl sulfide.

4. The method of claim 3 wherein the a ing solution also contains sodium sulfide.

1,921,539 2,099,400 11/1937 Jones 162-90 X 5. The method of claim 3 wherein the amount of carsulfide is from 0.01 to 2.0 parts by weight based on the sodium hydroxide.

queous digcst 45 DONALL H. SYLVESTER, Primary Examiner.

CAINE. Assistant Examiner, 

1. THE METHOD OF PREPARING WOOD PULP WHICH COMPRISES DIGESTING WOOD CHIPS IN AN AQUEOUS SOLUTION OF ALKALI METAL HYDROXIDE WHICH ALSO CONTAINS CARBONYL SULFIDE. 